The present patent application is directed to low density paper and paperboard and, more particularly, to low density paper and paperboard having a smooth, coated surface on both sides.
Paperboard is commonly used in various packaging applications. For example, high end personal care or commercial printing applications and the like. The paperboard often receives a variety of graphic treatments to enhance its visual impact on the shelf. Likewise, quality papers to be utilized as a medium for printing require smooth coated surfaces, with few imperfections to facilitate the printing of high quality text and graphics.
Conventionally, smoothness is achieved by calendering. Calendering serves to mechanically compress the sheet, providing a surface roughness low enough to produce final coated smoothness acceptable to the industry. However, this compression results in the severe densification of the sheet. Therefore, smooth papers and paperboard are typically more dense (i.e., less bulky) than less smooth paper and paperboard. This effect is magnified when a smooth, coated print surface is required on both sides of the paperboard.
For example, in FIG. 1, the basis weight in pounds per ream (1 ream=3000 ft.sup.2) of certain prior art coated two-side (C2S) solid bleached sulfate (SBS) paperboard products and C2S fine paper products is plotted against caliper thickness (1 point=0.001 inch=1 mil), thereby providing a visual representation of prior art paper and paperboard density (i.e., basis weight divided by caliper thickness). As can be seen, for a given caliper, the sheet will have typically been pressed to a given density range in order for the needed surface smoothness to be developed.
Nonetheless, low density is a desirable quality in many paper and paperboard applications. However, preparing a smooth surface using the conventional calendering process requires substantially increasing the density of the fiber substrate.
Accordingly, there is a need for a low density paper and paperboard that provides the desired smoothness on both sides for high quality printing, while reducing raw material cost.